Hydrogen bonding, protonation, and lithium ion association will be systematically investigated in ground and low-energy excited electronic states of series of heterocyclic nitrogen bases (such as purines, pyrimidines, and pyridines), and in carbonyl bases, by means of ab initio LCAO-MO-SCF and SCF-CI calculations. The particular bases chosen for investigation are those which contain the specfic chemical moieties that are present and important in biochemical systems, but which have not yet been investigated in detail by ab initio molecular orbital techniques. The structural, energetic, and electronic properties of these bases and the complexes which they form with H2O, H ions and Li ions will be determined, and correlations established between the properties of these bases and the structures and stabilization energies of their complexes. Substituent effects on base strengths, and nonadditivities of interaction energies in hydrogen bonded trimers and solvated protonated bases and lithium ion complexes, will be evaluated. The interactions of the nucleotide bases with H2O, H ion, and Li ion, and the pairing of nucleotide bases, will also be investigated. The results of this research will be used to analyze and characterize hydrogen bonding and ion-molecule associations in ground and excited electronic states of biochemical systems and to elucidate the implications for biochemical processes. In this way, the results of this research will contribute to future progress in biochemistry and molecular biology.